Under the influence of hydrothermal activity, the coal and its surrounding rocks in the Carboniferous-Permian coal-bearing measures in Henan Province have changed. The characteristics of surrounding rock changes and hydrothermal properties are studied, and it is considered that hydrothermal solution is groundwater hydrothermal solution formed by atmospheric precipitation. Based on the formation mechanism of groundwater hydrothermal system, the relevant viewpoints in hydrogeology are applied to the study of coal metamorphism, and it is pointed out that groundwater hydrothermal system is a convective heat medium in the process of coal thermal metamorphism. In addition, the change characteristics of clay minerals in surrounding rocks divide the change stages of surrounding rocks, which correspond to the metamorphism stages of coal.

Henan Carboniferous-Permian coalfield is located in the south of Carboniferous-Permian coal-accumulating depression in North China, centered on Jiyuan, Jiaozuo, Yanlong and Xinggong anthracite mining areas, and is famous for its large number of coal seams and obvious coal metamorphic zoning (Figure 1). Coal metamorphism is the cause of the formation of large area anthracite and high metamorphic bituminous coal in this area, and it has always been a subject that many coalfield geologists are committed to studying. In recent years, although some researchers have discovered the thermal metamorphic characteristics of Henan coal, and further confirmed that abnormal geothermal factors are of great significance to the formation of metamorphic zones, they have also reached a consensus. However, there are still different views on the heat source and the action mode of heat that leads to coal thermal metamorphism. This paper tries to apply the knowledge of groundwater hydrothermal in hydrogeology to the study of coal thermal metamorphism by studying the changing characteristics of surrounding rock under the condition of coal thermal metamorphism and considering the formation mechanism of geothermal anomaly.

A brief introduction to regional geology

The geological development history of the study area is similar to that of most parts of North China. Carboniferous and Permian strata are the most important coal-bearing strata in this area, with the maximum thickness of about 1500m, which is in contact with the underlying Ordovician or CAMBRIAN pseudo-integration. The maximum thickness of overlying continuous sedimentary caprock is about 3500m. The Permian Shanxi Formation 1 coal seam is the main minable coal seam in the whole region. Therefore, the horizontal zoning of coal metamorphism is mainly aimed at the second 1 coal seam.

According to various geotectonic viewpoints, central Henan and northern Henan are the junction of different tectonic units. Hehuai fault, which extends along Zhengzhou-Suxian line, traverses the whole area, and some deep faults in the near north-south direction intersect with it [1]. Fold structure is not developed in the whole region, and fault structure is the main structure. The more important regional faults are Wu Zhiling fault, Songshan fault and Xiangyang fault.

Figure 1 Isogram of Coal Reflectance of Shanxi Formation in Carboniferous-Permian Coalfield of Henan Province

Secondly, the alteration and mineralization of coal-bearing rock series

Through field and mine geological observation, it is found that the coal-bearing measures in the anthracite belt in this area are generally altered and mineralized. The main alteration types are silicification, calcite, chloritization and pyrophyllite.

Silicification and calcite make many time-dependent veins and calcite veins develop in coal-bearing rock series, and in some areas, the time-dependent veins are covered by clastic particles. The width of these gangue stones is mostly around 1cm, and the widest can reach more than 20cm. The length is several centimeters to several meters, even dozens of meters. They are filled along the open cracks in rocks and coal seams, and the surrounding rocks at the edge of veins often have narrow altered edges, and the mineral composition and color have changed to varying degrees. The appearance of gangue is consistent with the composition of surrounding rock. Time-dependent pulses are generally only found in the strata of Shanxi Formation and above rich in silica, and rarely appear in limestone of Taiyuan Formation. Calcite veins are common in Taiyuan Formation rich in CaCO3.

Some siderite veins, pyrite veins, chlorite veins and pyrophyllite veins are smaller than calcite veins and timely veins, and they are often associated with the latter.

Chloritization and pyrophyllite are mainly produced in rocks rich in clay minerals. Visually, they are often in the form of thin films and veinlets in argillaceous rocks, and often occur at the edge of time veins in sandstone, forming an altered edge of 1 to several millimeters. The film-like components are relatively pure, and the veins are often mixed with microcrystalline and star-like disseminated metal minerals. By powder X-ray diffraction analysis, chlorite has characteristic diffraction peaks D (001) =14.2×10-10m, and D (002) = 7./kloc-0 /×10. The diffraction peaks of pyrophyllite are d (002) = 9.15×10-10m, and d (004) = 4.6×10-/0m, and d (006).

The identification data of rocks and minerals in monster hunter and Xinggong mining areas show that chloritization and pyrophyllite can also be seen in argillaceous interstitial materials of sandstone and mudstone under microscope. In addition, the external cracks of massive anthracite in Jiyuan and Jiaozuo are also filled with chlorite and pyrophyllite films, and the chloritization and pyrophyllite of roof rocks are more intense.

Mineralization related to rock alteration has not been noticed by everyone. According to the author's observation, there are metal minerals in the chronological veins and calcite veins in Yanlong, Jiyuan, Xinggong and Jiaozuo. Pyrite, siderite, sphalerite, chalcopyrite, galena etc. It was identified by X-ray diffraction of mineral powder crystals. Pyrite and siderite are common, with good crystal form, and some of them form porphyritic crystals.

Generally speaking, the phenomenon of alteration and mineralization is mainly found in high metamorphic coal areas, such as Jiyuan, Jiaozuo, Yanlong, Xinggong and other mining areas, and the phenomenon of alteration and mineralization gradually disappears outward around this center.

3. Study on mineral inclusions in coal gangue.

Observe the contents through a thin sheet polished on both sides. There are many inclusions in timely calcite, mainly two-phase fluid inclusions, which are regularly oval and evenly distributed in the sample, and the gas-liquid ratio is generally 15% ~ 20%. The individual is large, generally 8 ~ 12 micron, and the maximum is over 30μ m. Generally, the liquid phase is light gray, the gas phase is gray, and occasionally the liquid phase is light pink. A few sporadic gas inclusions and CO2 inclusions were found in some samples. The inclusions in siderite are small, generally about 2 ~ 5 microns, and the shape is consistent with the crystal form of siderite. The liquid phase is light green, and the gas-liquid ratio is 10% ~ 15%. When all fluid inclusions in gas phase and liquid phase reach their uniform temperature, they are homogenized into liquid phase.

The results of temperature measurement of inclusions, chemical composition of inclusions and hydrogen-oxygen stable isotope analysis of water are listed in Table 1 and Table 2 respectively.

Table 1 Homogenization temperature and fracture temperature of fluid inclusions

sequential

Note: The temperature measurement of inclusions is completed by Beijing Institute of Uranium Geology of the nuclear industry.

Table 2 Chemical Composition of Inclusions and Water Stable Isotope Composition of Inclusions

Note: * The sample was analyzed by Guiyang Institute of Geochemistry, China Academy of Sciences; * * The sample was analyzed by Beijing Institute of Uranium Geology; /indicates that the component was not detected.

Fourthly, the change of clay minerals.

X-ray diffraction analysis of argillaceous interstitial materials in mudstone and sandstone in different mining areas shows that the clay mineral assemblage in different mining areas is quite different. Jiyuan, Jiaozuo, Yanlong and Xinggong samples are pyrophyllite, chlorite, illite and kaolinite, while Pingdingshan samples are mainly composed of irregular mixed clay minerals, illite and kaolinite.

Although illite is contained in samples from different habitats, the diffraction curve shows that its crystallinity is different. Using the method proposed by WE-BER (1972) [3], the crystallinity of illite is expressed by the relative half-width Hbrel of illite characteristic diffraction peak d(00 1), and under certain X-ray diffraction conditions, illite D (001) =1. that is

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The crystallinity of illite is generally expressed directly by the full width at half maximum of peak D (001) =10×10-10m. Because the peak width is easily influenced by instrumental analysis conditions, the crystallinity proposed by different researchers is not comparable. The crystallinity expressed by Hbrel can be compared with some typical examples abroad. The results of sample analysis are shown in Table 3. From the south to the north of Pingdingshan mining area, the crystallinity of illite increases with the increase of coalification degree. In addition, Pingdingshan 29- 15 sample is controlled by buried depth, and the crystallinity increases with the increase of buried depth.

Table 3 Illite crystallinity value Hbrel in several mining areas

Note: The samples are analyzed by the Experimental Center of Petroleum Exploration and Development Research Institute. * Clay mineral particles smaller than 2 microns ..

Discussion on verbs (abbreviation of verb)

(1) Relationship between alteration and mineralization of coal-bearing rocks and coal metamorphism

Silicification, calcite, chloritization and pyrophyllite are common medium-temperature alteration phenomena. Pyrite, siderite, sphalerite, chalcopyrite, galena etc. It is a common mesophilic mineral. By measuring the temperature of mineral inclusions, it is confirmed that the solution forming the time-dependent veins and calcite veins has the property of medium-high temperature hydrothermal solution, and the highest temperature reaches 200 ~ 350℃.

It is impossible to reach such a high paleogeothermal under normal geothermal gradient by accumulating the thickness of coal-bearing rock series and its overlying strata in this area. The formation of hydrothermal solution must have an abnormal geothermal field as the background, and this abnormal paleogeothermal temperature with hydrothermal solution is obvious for the transformation of coal-bearing rock series (including coal metamorphism).

Coarse cubic crystals formed by recrystallization of pyrite in limestone of Taiyuan Formation of Upper Carboniferous are common. Sometimes, pyrite veins even penetrate into coal seams. Figure 2 shows that the bottom coal seam of Taiyuan Formation in Qinyang Shuiyusi Coal Mine was replaced by pyrite. Under the microscope, a large number of pyrite fine crystals are dissolved and recrystallized into flakes, and the original crystal form of a single small cubic crystal is still faintly discernible. The temperature of calcite vein in limestone on the roof of the coal seam is 365℃ measured by blasting method, and a small amount of chalcopyrite exists in calcite vein.

Fig. 2 schematic diagram of pyritization of Taiyuan formation coal seam in Qinyang Shuiyusi coal mine

Under the microscope, Taiyuan Formation coal in Jiaozuo, Yanlong and other mining areas often contains pyrite, calcite and timely reticulated veins. In these reticulated veins, pyrite often occurs in the form of calcite or isochron, and these pyrite crystals may contain calcite or isochron crystals. This, like the example in Figure 2, reflects the activation, migration, settlement and recrystallization of deposited FeS2 under the action of abnormal paleotemperature and hydrothermal solution. There are few pyrite veins, reticular calcite veins and chronological veins containing pyrite in rocks and coal seams other than Carboniferous, so it can be judged that the migration range of FeS2 is limited. Two samples of lifeng mine in Jiaozuo mining area were measured, one was cubic crystal containing pyrite in the timely vein, and the other was massive pyrite mixed in the timely vein. The bursting temperature of the former is 135℃ and that of pyrite is 1 10℃. The explosion temperature of pyrite in the latter is 65438 030℃. It is proved that pyrite precipitates and recrystallizes at higher temperature.

As an organic rock, coal is more sensitive to high temperature than surrounding rock. The highest paleogeothermal temperature of 200 ~ 350℃ is enough to cause strong metamorphism of coal and form anthracite. The consistency of the distribution of high metamorphic zone and strong alteration zone also illustrates this point. Judging from the large-scale alteration phenomenon and high paleotemperature, the abnormal paleotemperature field is regional. Therefore, we attribute the large metamorphic zoning of Carboniferous-Permian coal in Henan Province to this abnormal paleogeothermal field. There has been a special article on the change characteristics of coal under hydrothermal action [4].

(2) The relationship between hydrothermal genesis and abnormal geothermal field formation.

In recent years, while pointing out the important influence of abnormal paleogeothermal on the formation of coal metamorphic zoning in Henan Province, some researchers are limited to the geological model of regional magmatic thermal metamorphism, trying to find hidden rock bodies, ignoring the further discussion on the formation mechanism of abnormal geothermal.

In fact, except for a few small rocks in Himalayan and Yanshan periods, no other igneous rocks have been found in the coalfield so far. The discovered rock masses are mainly distributed in mining areas outside the high metamorphic belt, and they have not been confirmed as geophysical anomalies caused by concealed rock masses. The aeromagnetic anomaly in this area is generally a reflection of the fluctuation of the top surface of Precambrian crystalline basement. Even if it is considered that there are some hidden rock masses surrounded by high abnormal values, their scale is not enough to produce magmatic hydrothermal solution that affects the whole area and make the area thermally activated. Therefore, the application of regional magmatic thermal metamorphism model of coal has encountered difficulties.

We have noticed that all kinds of veins found in the field are small in scale, but very common. Small and scattered is the distribution characteristic of gangue, and the composition of gangue is obviously consistent with the chemical composition of surrounding rock, which shows that minerals are mainly transferred from surrounding rock during hydrothermal seepage, and also shows that the genesis and evolution of hydrothermal solution are related to geological characteristics. Here, the genesis of hydrothermal solution actually involves the formation mechanism of geothermal anomaly. Water is the main component of hydrothermal solution, so studying the source and origin of water is the key to study the genesis of hydrothermal solution. The ultimate source of water can only be determined by studying some geochemical parameters of water molecules themselves, and stable isotopes of hydrogen and oxygen provide such parameters.

The research theory of mineral inclusions tells us that the primary inclusions are actually a part of the original hydrothermal solution preserved during the formation of minerals. Therefore, the source and evolution characteristics of water in hydrothermal solution can be judged by measuring the stable isotopic composition of hydrogen and oxygen in inclusion water. Referring to Sheppard's (1977) comprehensive map of isotopic composition of water from different sources [5], the author compiled Figure 3. It can be seen that the δD and δ 18O values of timely inclusion water in the study area fall near the rain line in the figure, and the δD value changes little, but δ 18O shows the characteristics of deviating from the rain line and moving in the higher δ 18O direction, which is generally considered to be the result of isotope exchange between hot water and silicate and carbonate surrounding rocks [6]. That is to say, the water in the hydrothermal solution that forms the time-dependent veins and calcite veins in this area mainly comes from rainwater (atmospheric precipitation). According to the source of water, this kind of hydrothermal solution is called groundwater hydrothermal solution, which is the most common hydrothermal system in the uppermost 2-4 miles (about 3-6.5 kilometers) of continental crust, and has been paid more and more attention by mineral deposit scientists [7, 8]. Of course, a single source of hydrothermal water is unthinkable, and we do not rule out that a small amount of water produced by deep metamorphism or magmatism will be added to this hydrothermal system.

The chemical composition of inclusions listed in Table 2 is not as complicated as magmatic hydrothermal solution. The anions in inclusion water are SO4 2-, HCO-3 and Cl-, and there is a trace of F- in Jiyuan sample, and the cations are K+, Na+, Ca++ and Mg++. The total salinity expressed by the sum of anions and cations is also low, ranging from 26.6 to 53.1mg/g, belonging to HCO-3-SO4-naca and Cl. This groundwater solution rich in oxygen, carbon dioxide and sulfate ions has strong erosion ability. In the process of infiltration, various elements rich in surrounding rock can be gradually dissolved into ore-bearing solution, and various gangue and metal minerals can be precipitated.

Figure 3 Stable Isotopic Composition of Inclusion Water | Figure 3 Stable Isotopic Composition of Inclusion Body Fluid | The dotted line indicates δ 18O migration caused by isotope exchange between hot water and rocks. The broken line explains that δ 18O migration is caused by heterogeneous exchange between hot water and rocks.

Obviously, atmospheric precipitation was heated in the process of entering underground seepage and evolved into hydrothermal solution. Geothermal and hydrogeological studies have proved that the movement of groundwater is the most active factor affecting the distribution of temperature field in the upper lithosphere. Groundwater circulating in the earth's crust has large heat capacity and good convective heat transfer performance. It is an ideal heat-carrying fluid in the earth's crust and an important medium for transferring underground heat energy from the deep to the surface [8, 9]. Geothermal anomalies caused by deep circulating heating of groundwater are widely distributed in crustal uplift and subsidence areas within the plate.

Of course, the temperature rise of this groundwater mainly depends on the underground heating system. We have noticed that the large-scale hydrothermal activity of groundwater is due to the thermal activation in the region, and the heat flow and geothermal gradient are generally improved, but this activation phenomenon can not be caused by the heat released by an intrusion. The lack of magmatic activity in this area also makes the basis for attributing regional thermal activation to magmatic intrusion insufficient. According to preliminary analysis, the "heating station" for increasing groundwater temperature may be related to the heat source formed by the late Mesozoic structural pattern change in southern North China and the deep Moho surface fluctuation.

Figure 1 The strike and distribution of coal metamorphic belt are obviously controlled by some deep and large faults, and the reflection isoline near important regional faults is distorted. Other coal rank parameters, such as volatile matter and H/C atomic ratio, change regularly from both sides of the fault to the direction of the fault, indicating that the fault may be used as a macro channel for deep heat flow upwelling and groundwater activity. Groundwater hydrothermal solution is essentially a heat transfer medium between deep heat source and coal-bearing rock series, and its interaction with surrounding rock includes alteration during seepage and heat exchange with surrounding rock, which heats coal-bearing rock series and promotes thermal metamorphism of coal. In this way, the thermal metamorphism period of Carboniferous and Permian coal in Henan Province, that is, the formation and activity period of groundwater hydrothermal solution, is equivalent to the structural pattern change period in southern North China and the fluctuation period of the upper mantle top interface in the late Mesozoic.

The micro-metamorphism characteristics of coal in this area are studied by various means, and the possibility of rock stratum and coal seam pores as micro-channels of groundwater hydrothermal seepage is confirmed [4, 10].

(3) the relationship between epigenetic change stage of surrounding rock and coal metamorphism stage.

The change of clay minerals is the result of paleogeothermal anomaly affected by groundwater hydrothermal solution. The epigenetic change of anthracite belt similar to this area has also been reported abroad [1 1, 12]. For example, the Upper Carboniferous in Belledore near Lorraine, France, the anthracite mining area in Pennsylvania, USA, and the Middle Carboniferous and Lower Cretaceous in Bramchi, Lower Saxony Basin, Federal Republic of Germany. Are related to abnormal paleotemperature.

In the stage of shallow metamorphism of rocks, the following equilibrium exists [1 1]:

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When the water pressure is 0. 1GPa, the formation temperature of pyrophyllite is about 350℃. In this equilibrium state, the activity of Si increases, the fugacity of water decreases and the equilibrium temperature decreases.

The increase of crystallinity of authigenic illite mainly depends on temperature, but has nothing to do with the degree of rock deformation. Hbrel value of rock illite crystallinity in this area reflects the increasing trend of crystallinity from Pingdingshan mining area to Jiyuan and Yanlong mining areas with strong hydrothermal activity. According to the above analysis, the apparent change stages of surrounding rock are preliminarily divided (Table 4).

The rocks of coal-bearing strata in anthracite belt in this area have entered the stage of shallow metamorphism. The development of pyrophyllite, the disappearance of montmorillonite, the disappearance of illite-montmorillonite irregular mixed layer and the dominance of illite and chlorite in clastic sedimentary rocks are all signs of shallow metamorphism.

The temperature range of shallow metamorphic stage is 200 ~ 350℃, which is consistent with the highest paleogeothermal range determined by hydrothermal mineral inclusion temperature measurement, indicating that it is possible to use illite crystallinity Hbrel to divide rock supergene change stages and judge paleogeothermal. Hbrel

Table 4 Relationship between Apparent Change Stage of Surrounding Rock and Coal Metamorphic Stage

* r max is the maximum reflectance of vitrinite in oil-immersed medium; R min is the minimum reflectance of vitrinite in oil-immersed medium; * * The wavy lines indicate that the boundaries are inconsistent.

This table is adapted from the article by teichüller et al. (1979). In the table, the crystallinity Hbrel of illite adopts the X-ray diffraction data of clay minerals less than 2 μ m..

Six, some understanding

(1) The Carboniferous-Permian coal-bearing series in Henan Province experienced hydrothermal alteration and mineralization in a large area, which is the direct evidence of paleogeothermal anomaly. According to the temperature measurement of hydrothermal mineral inclusions, the highest temperature of hydrothermal action is 200 ~ 350℃. This temperature range is suitable for the formation of anthracite in this area, which can be confirmed by the coincidence of the distribution of strong hydrothermal activity area and high metamorphic zone.

(2) Hydrothermal mineral inclusion water was studied by hydrogen and oxygen stable isotope analysis. The composition characteristics of δD and δ 18O values of inclusion water indicate that the water in hydrothermal solution mainly comes from atmospheric precipitation. There is no doubt that atmospheric precipitation is heated after entering the deep underground and evolved into groundwater hydrothermal solution.

(3) During the thermal metamorphism of Permo-Carboniferous coal in Henan Province, the groundwater hydrothermal solution, as a heat-carrying fluid, is the medium connecting the deep heat source with the coal-bearing rock series. It exchanges heat with surrounding rocks in the process of seepage along the pores of rock stratum and coal seam, and transfers abnormal heat to rock stratum and coal seam through convection, thus promoting the metamorphism of coal.

The hydrothermal metamorphism of groundwater controlled by deep fault pattern determines the trend and distribution of coal metamorphic belt in Henan Province.

(4) The change of mineral assemblage and the increase of illite crystallinity are also influenced by abnormal paleotemperature under hydrothermal action. The supergene change stages of rocks can be divided by clay mineral change and illite crystallinity, and then the paleotemperature of coalification can be determined.

In the past, the research on thermal metamorphism of coal emphasized the conduction and heat transfer mode of direct baking of magma heat, ignoring the influence of convection mode of heat-carrying fluid medium with higher heat transfer efficiency. Taking Henan as an example, combined with the formation mechanism of geothermal anomaly, a coal thermal metamorphism model with groundwater hydrothermal solution as heat transfer medium is put forward for further discussion and research.

The research work of this paper has been warmly guided by Professor Han Dexin and assisted by Henan Coalfield Geological Exploration Company and several coal mines in Henan Province. Thanks to the experimental center of petroleum exploration and development research institute, the laboratory of Henan Bureau of Geology and Mineral Resources, Beijing Institute of Uranium Geology of nuclear industry and Guiyang Institute of Geochemistry of China Academy of Sciences for their help in sample testing.

Take the exam and contribute.

[1] Zhang Wenyou, Zhao Yonggui, Xu, Wu, Han Beichuan, Zeng Xiangshan, 1983, Mesozoic-Cenozoic geological structure characteristics and lithospheric dynamic model in North China fault block area. Journal of Geology, vol. 57,No. 1, p. 33.

[2] Zhou Xingxi,, 1984, Discussion on Triassic distribution in southern North China Basin. Petroleum Realistic Geology, Volume 6, No.2, page 87

Weper, K, 1972, Notes on Determination of Small Crystals,-N.JB. Minerals, Mh, 2, pp. 267-276.

Ren Deyi, Zhong Ningning, Xiao Xianming, 1987, Micro-petrographic characteristics of regional thermometamorphic coal and its geological significance. Proceedings of the Symposium on Carboniferous and Permian Coal-bearing Strata and Geology in China, p. 359. Science press

Sheppard, S.M.F., 1977, Identification of the source of ore-forming solution by stable isotopes, volcanism in ore genesis (Special Issue No.7 of Geological Society of London).

[6] Liu, editor-in-chief, 1985, reference book of mineral deposit science (I). Page 206. Geological publishing house

Taylor, H.P., 1974, application of oxygen and hydrogen isotope research in hydrothermal alteration and ore deposition, economic geology, vol. 69, pp. 843-883.

[8] Shen, editor-in-chief,. 1985, Hydrogeology, pp. 723-778. Science press

[9] Huang Shangyao, Zheng Kebang, 1982, Geothermal Basic Theory. Page 1 ~ 30. Geological publishing house.

[10] Zhong Ningning, Telly, 1989, Preliminary application of electron probe method in coal petrology research. Coalfield Geology and Exploration, Phase 2, Page 17

[1 1] Kasch, I.J., 1983, Petrology and mineral petrology of burial diagnosis (burial metamorphism) and initial metamorphism in clastic rocks. The development of sedimentology. Volume 25B

[12] Teichmüller, m., Teichmüller, R., Weber, K., 1979, Inkohlung and illit kristallitat, veraleichende untersuchungen in mesosoikum and Palaozoikum von Westfalen. Porter Schell George Reinde. u. Westf。 27 Page 2065,438+0 ~ 276

Coal thermal metamorphism and alteration of Carboniferous-Permian coal-bearing measures in Henan Province

Preliminary discussion on the influence of underground hot water on coal metamorphism

Zhong Ningning

(Jianghan Petroleum Institute, Shashi, Hubei)

Ren Deyi

(Beijing Graduate School of China University of Mining and Technology, Beijing)

Abstract: The Carboniferous-Permian coal-bearing strata in Henan Province have undergone some changes due to the influence of thermal fluid activities. The author has studied the alteration characteristics and hydrothermal properties of surrounding rocks, and thinks that hydrothermal solution may be underground hot water formed by atmospheric precipitation. Focusing on the formation mechanism of underground hot water system, the author applies the relevant viewpoints of hydrogeology to the study of coal metamorphism, and points out that underground hot water plays the role of heat transfer (convection) medium in the thermal metamorphism of coal. In addition, the alteration characteristics of clay minerals in surrounding rocks are used to distinguish the alteration stages of surrounding rocks and correspond to the metamorphic stages of coal.

(This paper was co-authored by Zhong Ningning and Ren Deyi and originally published in Geological Review, Volume 36, No.2, 1990).